“You’ve got a problem,” Nadia replied.
“Why, something wrong out there?”
“No no.”
“Well good, because really it’s you guys who have the problem, and I wouldn’t want you to have more than one! A dust storm has started down in the Claritas Fossae region, and it’s growing, and coming north at a good rate. We think it’ll reach you in a day or so.”
“Isn’t it early for dust storms?” Arkady asked.
“Well no, we’re at Ls = 240, which is pretty much the usual season for it. Southern spring. Anyway, there it is, and it’s coming your way.”
He sent a satellite photo of the storm, and they studied their TV screen closely. The region south of Tharsis was now obscured by an amorphous yellow cloud.
“We’d better take off for home right now,” Nadia said after studying the photo.
“At night?”
“We can run the props on batteries tonight, and recharge the batteries tomorrow morning. After that we may not have much sunlight, unless we can get above the dust.”
After some discussion with John, and then with Ann, they cast off. The wind was pushing them east-northeast, and on this heading they would pass just to the south of Olympus Mons. After that their hope was to get around the north flank of Tharsis, which would protect them from the dust storm for at least a while.
It seemed louder flying at night. The wind’s rush over the fabric of the bag was a fluctuating moan, the sound of their engines a pitiful little hum. They sat in the cockpit, lit only by dim green instrument lights, and talked in low voices as they moved over the black land below. They had about 3,000 kilometers to go before reaching Underhill; that was about 300 hours of flying time. If they went round the clock, it would be twelve days or so. But the storm, if it grew in the usual pattern, would reach them long before then. After that . . . it was hard to tell how it would go. Without sunlight the props would drain the batteries, and then—”Can we just float on the wind?” Nadia said. “Use the props for occasional directional nudges?”
“Maybe. But these things are designed with the props as part of the lift, you know.”
“Yeah.” She made coffee and brought mugs of it up to the cockpit. They sat and drank, and looked out at the black landscape, or the green sweep of the little radar screen. “We probably ought to drop everything we don’t need. Especially those damned windmills.”
“It’s all ballast, save it for when we need the lift.”
The hours of the night wore on. They traded shifts at the helm, and Nadia caught an uneasy hour’s sleep. When she returned to the cockpit, she saw that the black bulk of Tharsis had rolled over the horizon ahead of them: the two northernmost of the three prince volcanoes, Ascraeus Mons and Pavonis Mons, were visible as humps of occluded stars, out at the edge of the world. To their left Olympus Mons still bulked well above the horizon, and taken with the other two volcanoes, it looked as if they flew low in some truly gigantic canyon. The radar screen reproduced the view in miniature, in green lines on the screen’s gridwork.
Then, in the hour before dawn, it seemed as though another massive volcano were rising behind them. The whole southern horizon was lifting, low stars disappearing as they watched, Orion drowned in black. The storm was coming.
It caught them just at daybreak, choking off the red in the eastern sky, rolling over them, returning the world to rusty darkness. The wind picked up until it swept them along in a muted roar from the land below. The view out the windows was of a few meters of swirling yellow dust, like a close-up of the clouds of Jupiter. Eddies twisted the dirigible’s frame and the gondola trembled and bounced.
They were lucky north was the direction they wanted to go. At one point Arkady said, “The wind should hopefully wrap around the north shoulder of Tharsis.”
Nadia nodded silently. They hadn’t gotten the chance to recharge the batteries after the night’s flight, and without sunlight the motors wouldn’t run too much longer. “Hiroko told me sunlight on the ground during a storm is supposed to be about fifteen percent of normal,” she said. “Higher there should be more. So we’ll get some recharge, but it’ll be slow. Could be that over the course of the day we might get enough to use the props a bit tonight.” She flicked on a computer to do the calculations. Something in the expression on Arkady’s face— not fear, not even anxiety, but a curious little smile— made her aware of how much danger they were in. If they couldn’t use the props, they wouldn’t be able to direct their movement, and they might not even be able to stay aloft. They could descend, it was true, and try to anchor, but they had only a few weeks’ more food, and storms like these often persisted for two months, sometimes three.